If the user cannot provide a snapshot of the rates (base rates at least), or if it is important
to get the latest Base Rates, then provide an implementation of this interface to the calculator.
Used in conjunction with the FxRateCalculatorBuilder.cacheBaseRates it is possible to request the base rates
(i.e. vs USD for instance) once or for every call.
Note: It is NOT the role of ObjectLabKit to provide an implementation for this, you can alternatively use
FxRateCalculatorBuilder.addRateSnapshot or FxRateCalculatorBuilder.ratesSnapshot to give the base rates to the calculator.